1. Field of the Invention
The present invention relates to a water control sheet, a gas diffusion sheet, a membrane-electrode assembly and a polymer electrolyte fuel cell, and to an independent water control sheet which has a shape retention property to be handled by itself, and a gas diffusion sheet, membrane-electrode assembly and polymer electrolyte fuel cell utilizing the water control sheet.
2. Related Background Art
On concern of depletion of oil resources, main challenges are to search for alternative fuels and to save resources regarding energy used in various forms. Among others, fuel cells which convert various fuels into chemical energy and produce electric power have been actively developed.
As described on page 5 of “Technical trend research on fuel cells” (Ed. Department of Technology Research, Japanese Patent Office, May 31, 2001, <URL> http://www.jpo.go.jp/shiryou/index.htm) [non-patent document 1], for example, fuel cells are classified into 4 groups depending on the type of electrolytes employed: phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs) and polymer electrolyte fuel cells (PEFCs). The operating temperature ranges of these types of fuel cells are limited depending on the electrolytes, and it has been known that PEFCs operate at a low temperature range of 100° C. or lower, PAFCs at a medium temperature range of 180° C. to 210° C., and MCFCs at a high temperature range of 600° C. or more and SOFCs at near 1000° C. Among these, common PEFCs which can operate in a low temperature range produce electric power incident to the chemical reaction between the fuels, hydrogen gas and oxygen-containing gas (e.g. air). As PEFCs allow production of electric power with relatively small apparatus configurations, their practical application has been urgently required.
FIG. 11 is a schematic sectional view of main parts of a fuel cell showing the fundamental configuration of conventional PEFCs. In this figure, components being constituted with substantially the same material or having substantially the same function are shown with the same hatchings. PEFCs comprise, as shown in FIG. 11, more than one cell unit stacked together, each unit comprising a membrane-electrode assembly (MEA) formed of a fuel electrode 17a, a polymer electrolyte membrane 19 and an air electrode 17c sandwiched between a pair of bipolar plates 11a and 11c. The fuel electrode 17a is composed of a catalyst layer 15a separating hydrogen into protons and electrons and a gas diffusion layer 13a providing fuel gas to the catalyst layer 15a, and between the catalyst layer 15a and the gas diffusion layer 13a is provided a water control layer 14a. On the other hand, the air electrode 17c is composed of a catalyst layer 15c in which reaction between protons, electrons and oxygen-containing gas occurs and a gas diffusion layer 13c providing oxygen-containing gas to the catalyst layer 15c, and between the catalyst layer 15c and the gas diffusion layer 13c is provided a water control layer 14c. 
The bipolar plate 11a has grooves for providing fuel gas. Fuel gas supplied through the grooves of the bipolar plate 11a diffuses through the gas diffusion layer 13a, is penetrated through the water control layer 14a and reaches the catalyst layer 15a. The supplied fuel gas is separated into protons and electrons, among which protons travel through the polymer electrolyte membrane 19 and reaches the catalyst layer 15c. On the other hand, electrons travel through an external circuit (not shown) and move to the air electrode 17c. Meanwhile, the bipolar plate 11c has grooves for providing oxygen-containing gas. The oxygen-containing gas supplied through the grooves of the bipolar plate 11c diffuses through the gas diffusion layer 13c, is penetrated through the water control layer 14c and reaches the catalyst layer 15c. The supplied oxygen-containing gas reacts with protons traveled through the polymer electrolyte membrane 19 and electrons moved through the external circuit to produce water. The produced water is ejected from the fuel cell through the water control layer 14c. Reverse-diffused water from the air electrode is also ejected from the fuel cell through the water control layer 14a at the fuel electrode.
The gas diffusion layers 13a and 13c and the water control layers 14a and 14c are required to have, under low moisture conditions, a moisture retaining property in order to maintain the polymer electrolyte membrane 19 wet and under high moisture conditions, a drainage property in order to prevent retention of water and flooding in the fuel cell. The conventional gas diffusion layers 13a and 13c and water control layers 14a and 14c are prepared by impregnating an conductive porous substrate such as a carbon paper with a fluororesin such as polytetrafluoroethylene or applying a paste of carbon powder mixed with a fluororesin on the conductive porous substrate, thereby forming the water control layers 14a and 14c as a region where the fluororesin or the carbon powder and fluororesin are present and the gas diffusion layers 13a and 13c as a region where this(these) material(s) is(are) not present. However, the thus-formed water control layers 14a and 14c may easily have decreased drainage and gas diffusion properties because excess fluororesin or carbon powder and fluororesin are impregnated to the conductive porous substrate. The water control layers 14a and 14c prepared according to such method also have low water and gas permeability in the planar direction (the direction perpendicular to the thickness), so as to cause reduction in fuel cell performances under the situation where high amount of water is produced due to their decreased drainage and gas diffusion properties.
Another method for preparing water control layers has been suggested in which a substrate is coated with a dispersion of a fluororesin and carbon to prepare a coated film and then the film (water control layer) is fixed on an conductive porous substrate by applying pressure, by which method the water control layer is prevented from impregnation (Japanese Patent Application Laid-open No. 2006-318790) [patent document 1]. However, this water control layer has insufficient drainage and gas diffusion properties because the layer formed is dense due to coating.
Still another method for preparing water control layers has been known in which electrospun nanofibers are formed or stacked on an conductive substrate and then the substrate is calcinated to form a carbonized nanofiber layer on the substrate (Japanese Patent Application laid-open No. 2007-273190 [patent document 2] and Japanese Patent Application laid-open No. 2008-201106 [patent document 3]). However, the obtained carbonized nanofibers are hard and brittle, deteriorating the handling characteristics. In addition, productivity is low with high cost due to calcination after formation of the nanofibers, making the process impractical.
In order to improve efficiency of calcination, it is suggested to electrospin a carbon black dispersed polymer material-containing solution to form a deposition layer before irradiation of a microwave to obtain carbon fibers (WO 2006/054636) [patent document 4]. However, this method also has, in addition to the similar problems as above patent documents 2 and 3 (Japanese Patent Application laid-open No. 2007-273190 and Japanese Patent Application laid-open No. 2008-201106), such problems that the drainage property is not sufficient due to the absence of a hydrophobic resin so that flooding easily occurs.
The present applicant has suggested a water control sheet in which a fluororesin and/or an conductive agent is loaded on a non-carbonized porous substrate sheet which has been formed without carbonization (Japanese Patent Application Laid-open No. 2010-192361) [patent document 5]. Although this water control sheet has such superior handling characteristics that it can be handled by itself, loading of the fluororesin and/or conductive agent may cause insufficient drainage and gas diffusion properties.